For example, as specified in MIL-STD-188-110B of Non-patent Document 1, in baseband signal processing on the transmission side, a modulation bit sequence x(m) (m being a symbol number in a time direction) subjected to error correction encoding and interleaving is converted into a symbol mapping number N(m). Then, it is scrambled using a predetermined scramble code NSCR(m), and converted into a transmission symbol number NTX(m). Then, it is subjected to phase-shift keying (PSK) modulation or quadrature amplitude modulation (QAM) based on the mapping rule for data transmission.
In addition, in Appendix C of Non-Patent Document 1, QPSK (quadrature-PSK), 8PSK, 16QAM, 32QAM, and 64QAM are defined as modulation schemes to be used, and scrambling is defined as follows.
In PSK, scrambling is represented by the following equation:NTX(m)=mod(N(m)+NSCR(m),8)  Eq. 1
where mod(A, B) is a remainder obtained by dividing A by B. FIG. 1 shows the transmission symbol mapping number of PSK and its arrangement. It can be seen from FIG. 1 and Eq. 1 that PSK scrambling is phase rotation by (π/4)*NSCR(m).
In QAM, scrambling is represented by the following equation:NTX(m)=bitxor(N(m),NSCR(m))  Eq. 2
where bitxor(A, B) is an operation of converting A and B into binary numbers and performing XOR (exclusive OR) for each bit. As in the above equation, QAM scrambling is a process in which symbol mapping points are irregularly replaced, rather than simple phase rotation in PSK. Further, as shown in FIG. 2, the QAM symbol mapping points of MIL-STD-188-110B Appendix C are arranged in a shape (12/4 star QAM) which is not a general lattice shape.
When performing hard decision in equalization decision processing, descrambling may be performed on the hard-decided received bit sequence by performing a reverse operation of the scrambling of each modulation scheme.
In Patent Document 1, there has been proposed a method in which a transmission side baseband signal processing unit performs phase rotation on the symbol-mapped signal by (π/4)*NSCR (M), and a reception side baseband signal processing unit performs phase rotation on the received detection signal by −(π/4)*NSCR(m) to perform a process corresponding to descrambling, or performs phase rotation on the mapped reference point signal by (π/4)*NSCR(m), or all the combinations of symbol mapping points that can be obtained using the reference point signal symbol mapping number N(m) and the scramble code NSCR(m) can be stored in advance in a read-only memory (ROM) or the like and referred to.
Further, as the QAM descrambling method, there is a method that is faithful to the principle of calculating a bit likelihood and then performing code conversion (equivalent to XOR operation) with reference to the scramble code.
Patent Document 1: Japanese Patent Application Publication No. 2013-42444
Non-patent Document 1: United States Department of Defense, “Department of Defense, Interface Standard MIL-STD-188 110B Interoperability and Performance Standards for Data Modems”, [online], 2000, [searched on Sep. 8, 2014], Internet<URL http://www.vocal.com/wp-content/uploads/2012/05/milstd_188110b.pdf>
However, in a demodulation process for QAM as in Non-patent Document 1, descrambling cannot be realized by simply manipulating an amplitude and/or a phase. Also, when dealing with a soft decision value and/or likelihood, it is necessary to calculate the likelihood in the case where all the symbol points are descrambled. In order to be able to process the QAM of Non-patent Document 1 in a wireless receiver of Patent Document 1, a correspondence table of all the combinations that can be obtained from all reference points and all scramble patterns is stored in the ROM and referred to, which also increases the amount of memory used, and is not preferable for implementation.
Further, in the case of using a conventional technique, in the bit likelihood calculation, two processes of phase rotation of PSK and bit code conversion of QAM are required, and the functions of implementation of the two processes and process selection are necessary.
Also, the same problem arises in the generation of the reference point which is necessary in a calculation process of a soft symbol calculation process in turbo equalization.